Unlimited rich ore that filled ancient vessels (5)
I believe the answer is:
ichor
'that filled ancient vessels' is the definition.
(bodily fluid of the Greek gods)
'unlimited rich ore' is the wordplay.
'unlimited' indicates a hidden word (the answer is currently being held or limited).
'ICHOR' is hidden in the letters of 'rich ore'.
(Other definitions for ichor that I've seen before include "Blood of the gods found in choir" , "Juice of the veins of the gods" , "Heavenly juice" , ""Blood" of Greek gods" , "Fluid in Greek gods' veins" .)